Update readme with usage instructions (#40)

Author: jwilger

README.md

@@ -10,6 +10,129 @@ of your domain events. It is built on top of [Kurrent](https://kurrent.dev/)
 (formerly EventStoreDB) and provides a clean, type-safe API for implementing
 event-sourced systems.
 
+## Getting Started
+
+### Installation
+
+Add Mneme to your `Cargo.toml`:
+
+```toml
+[dependencies]
+mneme = "0.1.0"
+```
+
+### Basic Concepts
+
+Mneme implements the event sourcing pattern with these core components:
+
+- **Commands**: Operations that may produce events
+- **Events**: Facts that have happened in the system
+- **State**: Derived from applying events sequentially
+- **Event Store**: Persists the event stream for each aggregate
+
+### Usage Example
+
+```rust
+use mneme::{AggregateState, Command, Event, EventStore, EventStreamId, execute};
+use serde::{Deserialize, Serialize};
+use uuid::Uuid;
+
+// 1. Define your events
+#[derive(Debug, Clone, Deserialize, Serialize)]
+enum BankAccountEvent {
+    Created { id: Uuid, owner: String },
+    Deposited { id: Uuid, amount: u32 },
+    Withdrawn { id: Uuid, amount: u32 }
+}
+
+impl Event for BankAccountEvent {
+    fn event_type(&self) -> String {
+        match self {
+            BankAccountEvent::Created { .. } => "BankAccount.Created".to_string(),
+            BankAccountEvent::Deposited { .. } => "BankAccount.Deposited".to_string(),
+            BankAccountEvent::Withdrawn { .. } => "BankAccount.Withdrawn".to_string(),
+        }
+    }
+}
+
+// 2. Define your aggregate state
+#[derive(Clone, Debug)]
+struct AccountState {
+    balance: u32,
+}
+
+impl AggregateState<BankAccountEvent> for AccountState {
+    fn apply(&mut self, event: &BankAccountEvent) -> &Self {
+        match event {
+            BankAccountEvent::Created { .. } => {},
+            BankAccountEvent::Deposited { amount, .. } => {
+                self.balance += amount;
+            },
+            BankAccountEvent::Withdrawn { amount, .. } => {
+                self.balance -= amount;
+            }
+        }
+        self
+    }
+}
+
+// 3. Define a command
+#[derive(Clone)]
+struct WithdrawCommand {
+    id: Uuid,
+    amount: u32,
+    state: AccountState,
+}
+
+impl Command for WithdrawCommand {
+    type Event = BankAccountEvent;
+    type State = AccountState;
+    type Error = String;
+
+    fn get_state(&self) -> Self::State {
+        self.state.clone()
+    }
+
+    fn set_state(&mut self, state: &Self::State) {
+        self.state = state.clone();
+    }
+
+    fn event_stream_id(&self) -> EventStreamId {
+        EventStreamId(self.id)
+    }
+
+    fn handle(&self) -> Result<Vec<Self::Event>, Self::Error> {
+        if self.amount <= self.state.balance {
+            Ok(vec![BankAccountEvent::Withdrawn { 
+                id: self.id, 
+                amount: self.amount 
+            }])
+        } else {
+            Err("Insufficient funds".to_string())
+        }
+    }
+}
+
+// 4. Use the execute function with your event store
+async fn process_withdrawal(account_id: Uuid, amount: u32) -> Result<(), mneme::Error> {
+    let mut event_store = /* your event store implementation */;
+    
+    let command = WithdrawCommand {
+        id: account_id,
+        amount,
+        state: AccountState { balance: 0 }, // Initial state will be replaced by stored events
+    };
+    
+    execute(command, &mut event_store, Default::default()).await
+}
+```
+
+## Advanced Features
+
+- **Optimistic Concurrency**: Handles concurrent updates to the same event stream
+- **State Reconstruction**: Automatically rebuilds aggregate state from event history
+- **Type Safety**: Leverages Rust's type system for safe event handling
+
 ## License
 
 This project is licensed under the MIT License - see the [LICENSE](LICENSE)